A transfer resin molding method has been conventionally adopted as a method of resin molding electronic components, e.g. with a thermosetting resin. A device for practicing the known method is shown in FIG. 1 as an example. Referring to FIG. 1, the known device comprises an upper mold 1, a lower mold 2 facing the upper mold 1, and a so-called pot 3 for supplying resin, provided in the lower mold 2. A plunger 4 for pressurizing the resin is inserted in the pot 3. Upper mold cavities 5a and lower mold cavities 5b are both provided for seal molding electronic components with resin. These cavities face each other along parting line faces, hereinafter referred to as PL faces, where the upper mold 1 abuts against the lower mold 2. A path 9 for transferring melted resin, includes a cull portion 6, a runner portion 7 and a gate portion 8. These portions are located between neighboring upper mold cavities 5a and face the PL face of the upper mold 1.
The resin-seal molding of the electronic components is carried out as follows by the known device. First, with the upper mold 1 and the lower mold 2 open, a lead frame 11 on which an electronic component 10 is mounted, is fitted or set in a predetermined position in a groove portion 12 formed on the PL face of the lower mold 2. At the same time, a resin tablet 13a is supplied into the pot 3. In this state, the lower mold 2 is raised toward and clamped to the upper mold 1. At this time, the resin tablet 13a in the pot 3 is melted by heaters 14 provided in the upper and lower molds 1 and 2. Thus, when the resin tablet 13a is pressured by the plunger 4, the melted resin is injected under pressure and filled into the upper and lower cavities 5a and 5b through the transfer path 9 from the pot 3. Then, after a required curing time, the lower mold 2 is lowered again to open the mold. Resin moldings 15 in the upper and lower cavities 5a and 5b and resin moldings 16 in the transfer path 9 are released from the mold by ejector pins 17 slidable in the upper and in the lower molds 1 and 2. The unnecessary resin oldings 16 and unnecessary portions of the lead frame 11 are then removed. The resulting product is the seal molded electronic component 10 in the resin moldings 15 corresponding to the configuration of the upper and lower cavities 5a and 5b.
The above described resin tablet 13a is produced by molding a fixed amount of resin powder under compression to form a cylindrical shape with a predetermined length. The main purposes of such a tablet has been conventionally to simplify the process of transferring and supplying resin into the pot 3 and to make it convenient to preheat the resin prior to the resin-seal molding. For the tablet it is merely necessary to retain a predetermined shape. Therefore, enhancements in hardness and compression density have usually not been required for the tablet. Further, the size and configuration of the tablet are individually determined by the size of the pot 3, the required amount of resin and the like.
A conventional tablet molding device has the same structure as a tablet machine which molds a tablet by pressing and hardening granules or powder. A schematic structure of a tablet molding device is shown in FIGS. 2A and 2B. The tablet molding device comprises a cylindrical compression mold 19 as a container for holding resin powder 18, and punches 20 for pressurizing the resin powder 18 in the compression mold 19. A conventional tablet is formed simply by compression of the resin powder 18 by the described tablet molding device in order to retain the predetermined shape conventionally required for the tablets. Thus, the following problems are involved in resin-seal molding employing tablets made as described above.
A large number of small pores are present within conventionally made resin tablets 13a, which contain approximately 20% of air in volume ratio and sometimes the known tablets even contain water, for example, by absorption of water after the molding. Therefore, when the resin tablet 13a containing air and water is supplied into the pot 3 and melted by heating, a large amount of air is present in the melted resin. This mixed-in air and moisture results in the formation of a void or voids on the surface of or within the resin-seal moldings 15, whereby the water resisting property and the mechanical strength are reduced or the appearance of the product deteriorates. Furthermore, since the resin-seal moldings 15 tend to have a smaller thickness when miniaturization of the electronic component 10 is involved, and since the produced void is a defective portion formed of the pores caused by the above mixed-in air, the deterioration of the water resisting property and of the mechanical strength causes a serious problem because the quality and reliability of the known product are substantially reduced.
Increasing the forming pressure of the punches 20 applied to the resin powder 18, provides a slight increase in the compression density of the resin tablet 13a to be molded. However, the specific gravity of the tablet to be molded by the above described conventional tablet molding device is limited to about 90% of a net or theoretically possible specific gravity of the resin, (hereinafter referred to as "90% in compression density"). Here, the "net specific gravity" represents an inherent specific gravity of the resin in the tablet not containing any air nor water, i.e., a density that cannot be increased even if more pressure is applied to the resin.
It is impossible to eliminate various problems resulting from the generation of the void when the compression density is equal to or less than 90%.
Moreover, even though it is intended to increase the compression density of the resin tablet 13a by applying a higher molding pressure, due to the presence of silica as filler in the resin for making seal moldings in large quantities there occurs an intense abrasion of the tools because the silica is very hard. Even when employing a super hard alloy for the punches of the tablet molding device the abrasion cannot be avoided.